1. Field of the Invention
The present invention relates to a processing apparatus having a plurality of processing units for processing a workpiece such as a semiconductor wafer and an optical device wafer.
2. Description of the Related Art
In a semiconductor device fabrication process, a plurality of crossing division lines called streets are formed on the front side of a substantially disk-shaped semiconductor wafer to thereby partition a plurality of rectangular regions where a plurality of semiconductor circuits are respectively formed. The semiconductor wafer thus having the plural semiconductor circuits is cut along these streets to thereby divide the rectangular regions where the semiconductor circuits are formed from each other, thus obtaining a plurality of individual semiconductor chips. Also in an optical device wafer such that a plurality of crossing streets are formed on the front side of a sapphire substrate or the like to partition a plurality of regions where a plurality of optical devices are respectively formed from gallium nitride compound semiconductors or the like, the optical device wafer is divided along the streets to obtain the individual optical devices such as light emitting diodes and laser diodes, which are widely used in electric equipment.
Prior to cutting the wafer along the streets to obtain the individual chips, the back side of the wafer is ground to reduce the thickness of the wafer to a predetermined thickness, thereby reducing the size and weight of each chip. In general, the grinding of the back side of the wafer is performed by using a grinding apparatus including coarse grinding means and finish grinding means for grinding a workpiece held on a chuck table. However, a time period for coarse grinding by the coarse grinding means is different from that for finish grinding by the finish grinding means, so that it is difficult to synchronize the coarse grinding operation and the finish grinding operation. To solve this problem, there has been proposed a grinding apparatus including semifinish grinding means in addition to the coarse grinding means and the finish grinding means (see Japanese Utility Model Laid-open No. Hei 2-15257, for example).
Further, when the back side of the wafer is ground, a grinding strain such as microcrack is produced on the back side of the wafer, causing a considerable reduction in die strength of each chip obtained by dividing the wafer. To remove the grinding strain produced on the back side of the wafer ground, there has been proposed a processing apparatus including polishing means in addition to the coarse grinding means, the semifinish grinding means, and the finish grinding means (see Japanese Utility Model Laid-open No. Hei 4-2776, for example).